This chapter summarizes Peirce’s attainments and accomplishments during the period of his scientific career, noting their variety, describing his important contributions to mathematical logic, and mentioning his philosophical essays and lectures, but emphasizing his empirical investigations, which were in experimental psychology as well as in astronomy and geodesy. He improved on such recent developments as Venn’s frequency theory of probability and Boole’s algebra of logic. He studied the histories of science and of logic. I report nothing not already known; this chapter’s purpose is to establish the depth of Peirce’s immersion in modern science, so as to ground the fundamental thesis of this book, that his philosophical modus operandi was that of a scientist. His essays should be read, not as constructing a system, much less as conceptual analysis, but as framing bold conjectures intended to guide the inquiries in which they will be refined and tested, amended or superseded.